There exists a system of dense acetylcholinesterase (ACHE) staining cholinergic cells in the rat forebrain which has been designated as the magnocellular nuclei of the basal forebrain (MNBF). Because of the diffuseness of the localization of the MNBF, no single lesion of this cell system can reveal the extent of the cholinergic projections throughout the brain. Therefore, a parceling of this chain of cells into segments that are appropriate for stereotaxic lesions seems most feasible. This study was undertaken to elucidate the cholinergic projections emanating from a specific portion of the MNBF, the ventral aspect of the nucleus of the tractus diagonalis (td). Following lesions choline acetyltransferase (ChAT) was used as the marker to determine cholinergic nerve projection sites. Significant decreases in ChAT activity were observed in the cingulate, frontal and occipital cortices, the hippocampus and dentate gyrus. This study supports the notion that a cholinergic system of cell bodies is contained in the basal forebrain which projects topographically to the neocortex, hippocampus and amygdala.